RANGER-4388: KMS metrics good coding practice review comments incorpo…

[vikaskr22]

…ration

What changes were proposed in this pull request?

Few review comments were provided on KMS API metrics implementation but after PR merge. This JIRA addresses those review comments.
Review comments: https://reviews.apache.org/r/74396/
All comments has been incorporated except one, that is to rename the API metrics. It may break backward compatibility if any product is consuming these metrics.

How was this patch tested?

-mvn clean install worked

• Manually hit all KMS REST end points (Docker setup) and captured the metrics json response and verified. Verified using some custom logger as well.
  Sample metrics response:

{ "KMS" : { "GET_CURRENT_KEY_COUNT" : 1, "DELETE_KEY_ELAPSED_TIME" : 41, "EEK_DECRYPT_ELAPSED_TIME" : 61, "GET_KEYS_METADATA_ELAPSED_TIME" : 30, "EEK_GENERATE_ELAPSED_TIME" : 85, "GET_CURRENT_KEY_ELAPSED_TIME" : 46, "EEK_REENCRYPT_ELAPSED_TIME" : 0, "KEY_CREATE_COUNT" : 2, "UNAUTHORIZED_CALLS_COUNT" : 0, "KEY_CREATE_ELAPSED_TIME" : 189, "GET_KEY_VERSION_COUNT" : 4, "ROLL_NEW_VERSION_ELAPSED_TIME" : 83, "REENCRYPT_EEK_BATCH_COUNT" : 0, "REENCRYPT_EEK_BATCH_ELAPSED_TIME" : 0, "GET_KEYS_METADATA_COUNT" : 1, "GET_KEY_VERSIONS_COUNT" : 0, "GET_KEY_VERSIONS_ELAPSED_TIME" : 0, "GET_KEYS_COUNT" : 5, "EEK_GENERATE_COUNT" : 2, "INVALIDATE_CACHE_COUNT" : 1, "GET_METADATA_COUNT" : 6, "REENCRYPT_EEK_BATCH_KEYS_COUNT" : 0, "EEK_REENCRYPT_COUNT" : 0, "UNAUTHENTICATED_CALLS_COUNT" : 0, "GET_KEY_VERSION_ELAPSED_TIME" : 45, "INVALIDATE_CACHE_ELAPSED_TIME" : 14, "ROLL_NEW_VERSION_COUNT" : 1, "EEK_DECRYPT_COUNT" : 3, "GET_KEYS_METADATA_KEYNAMES_COUNT" : 0, "DELETE_KEY_COUNT" : 1, "GET_KEYS_ELAPSED_TIME" : 136, "GET_METADATA_ELAPSED_TIME" : 177, "TOTAL_CALL_COUNT" : 40 }, "RangerWebContainer" : { "ActiveConnectionsCount" : 1, "ConnectionTimeout" : 60000, "MaxWorkerThreadsCount" : 200, "TotalWorkerThreadsCount" : 10, "KeepAliveTimeout" : 60000, "ActiveWorkerThreadsCount" : 0, "ConnectionAcceptCount" : 100, "MinSpareWorkerThreadsCount" : 10, "MaxConnectionsCount" : 8192 }, "RangerJvm" : { "GcTimeTotal" : 795, "SystemLoadAvg" : 0.02, "ThreadsBusy" : 5, "GcCountTotal" : 10, "MemoryMax" : 1008205824, "MemoryCurrent" : 251048424, "ThreadsWaiting" : 25, "ProcessorsAvailable" : 5, "GcTimeMax" : 795, "ThreadsBlocked" : 0, "ThreadsRemaining" : 16 }

CC: @mneethiraj